The removal of sulfur dioxide from hot gaseous streams, such as hot combustion gases resulting from the burning of coal, has been effected by various wet scrubbing processes using as a scrubbing medium lime, magnesium-enhanced lime, or magnesium oxide or hydroxide. Wet scrubbing processes using magnesium-enhanced lime as a scrubbing medium are described, for example, in U.S. Pat. No. 3,919,393, U.S. Pat. No. 3,919,394, U.S. Pat. No. 3,914,378, and U.S. Pat. No. 4,976,936, all of which are assigned to the assignee of the present invention.
Another type of wet scrubbing system for removing sulfur dioxide from a hot gaseous stream uses a clear solution of magnesium scrubbing components without the presence of lime in the wet scrubbing unit. Such a system is described, for example, in U.S. Pat. No. 4,996,032, U.S. Pat. Nos. 5,039,499, 5,084,255 and U.S. Pat. No. 5,270,026, all of which are assigned to the assignee of the present invention, and the contents of all of which are incorporated by reference herein. In U.S. Pat. No. 5,039,499, a process is described wherein magnesium hydroxide is added to an aqueous scrubbing liquor to remove sulfur dioxide, with the scrubber effluent oxidized and then treated with a magnesium-containing lime slurry so as to regenerate magnesium hydroxide which is recycled to the wet scrubbing unit. U.S. Pat. No. 4,996,032, U.S. Pat. No. 5,084,255, and U.S. Pat. No. 5,270,026 teach additional treatment and recovery of components of the effluent from such a magnesium scrubbing system, so as to provide by-products in a more purified form. The provision of pure byproducts from such a magnesium scrubbing system encounters difficulties in that hydrocloning of by-product gypsum and magnesium hydroxide, resulting from the regeneration step, achieves a by-product containing about 70 percent magnesium hydroxide and 30 percent gypsum (CaSO.sub.4 .multidot.2H.sub.2 O). Use of fresh water to dissolve the gypsum with subsequent separation of the magnesium hydroxide therefrom can produce a magnesium hydroxide by-product of a purity of about 91 percent. With such dissolution of gypsum, however, the magnesium hydroxide produced does not have good dewatering properties, with thickening provided a magnesium hydroxide solids content of only about 15 percent and filtering providing a magnesium hydroxide solids content of only about 30 percent. It would be very beneficial if a higher purity and higher solids content of magnesium hydroxide by-product from flue gas desulfurization systems could be achieved since the higher the purity and the higher the solids content, the more value that is attached to the product.
In the oxidation step, for example, where magnesium bisulfite and magnesium sulfite that are present in the bleed stream from the wet scrubbing system are oxidized to magnesium sulfate, which is subsequently reacted with lime to produce calcium sulfate and magnesium hydroxide, in order to guarantee high magnesium hydroxide purity the final sulfite concentration must be less than about 100 ppm (parts per million). Magnesium sulfite, when reacted with lime in the regeneration step, produces very fine crystals of calcium sulfite which can contaminate the magnesium hydroxide product. Also, while it is relatively easy to oxidize the magnesium sulfite to an extent which provides about 500-600 ppm residual magnesium sulfite in the magnesium sulfate produced, a stoichiometric ratio of about 2:1 (moles of oxygen used vs. moles of oxygen theoretically required) is needed for conversion to a resultant magnesium sulfite level of about 500 ppm. In order to provide a residual level of magnesium sulfite in the magnesium sulfate produced which would be about 100 ppm or below, the optimal value, a stoichiometric ratio of about 5:1 would be required, with the attendant cost and energy factors associated therewith. Thus, the power expenditure would be 2.5 times as much to provide a 100 mg magnesium sulfite level vs a 500 ppm magnesium sulfite level in the magnesium sulfate reporting from the oxidation step of the process.
In the magnesium scrubbing system, of which the present process is an improvement, about ninety to ninety-five percent of the magnesium hydroxide produced is returned to the scrubbing system, such as to the recycle tank of the wet scrubbing unit and thus does not have to be of high purity and may contain residual calcium sulfites without interfering with the scrubbing efficiency. It is only when a purified magnesium hydroxide is desired, for sale as a by-product of the process, that a level of sulfite of below 100 ppm is desired.
It is an object of the present invention to provide a process for removing sulfur dioxide from a gaseous stream using an aqueous solution of magnesium scrubbing components such as to produce a pure magnesium hydroxide by-product.
It is another object of the present invention to provide a process for removing sulfur dioxide from a gaseous stream using an aqueous solution of magnesium scrubbing components where a purified magnesium hydroxide by-product is produced while minimizing power costs required and reducing the size of the oxidizing unit necessary.